In known apparatus of the type described above, problems arise in certain refining processes in view of the fact that an overpressure is created between the refining surfaces which can disturb the material-feeding process. At times this overpressure between the refining surfaces can undesirably prevent the lignocellulose-containing material from being subjected to the necessary processing pressures. In other cases, this overpressure can in some other ways disturb the refining process. Furthermore, pressure variations between the refining surfaces can occur, and thereby affect the process by disturbing the stability of the gap therebetween.
During the refining of wood chips, for example, where pieces of cellulosic material are centrally charged between two contra-rotating refining discs, the wood chips generally contain water, since they are usually steamed with hot water and/or steam. Some water is also added in connection with the refining process itself. Because during the refining operation only a portion of the energy supplied is consumed by the refining process itself, large amounts of steam are produced from this water. The resulting steam pressure then renders it difficult to charge additional chips when this steam must flow outwardly through the same inlet through which the chips are charged. Moreover, steam shocks occur and adversely affect the refining process by disturbing the gap stability.
In certain conventional refiners, such as that shown in Swedish Patent Application No. 7701182-3, which corresponds to U.S. patent application Ser. No. 656,003, now abandoned, which is the parent of application Ser. No. 865,176, which issued as U.S. Pat. No. 4,221,343 on Sept. 9, 1980, channels are provided through one of the refining discs to conduct steam away from the space between the refining discs. The steam is then permitted to flow from the rear side of the refining disc within the refiner housing to the outside of the housing through a stationary steam-collecting device provided in the housing. In this type of structural design significant problems arise, however, particularly with respect to the maintenance of a seal between the rotating discs and the stationary steam-collecting device. Such a seal must then be located within the refiner housing, which results in a considerable risk of clogging, while at the same time this renders it difficult to obtain access to the seal, such as for cleaning purposes. Furthermore, such a steam-collecting device has a disturbing effect on the flow of the refined stock in the refiner housing, rendering it difficult to axially move the refiner discs, and thereby difficult to obtain access when, for example, there is a need to exchange the refining surfaces (refining segments) of the refiner discs. It is an object of the present invention to eliminate the aforesaid disadvantages.